Heavy Movable Structure Health Monitoring: Case Study on a Bascule Bridge in Ft. Lauderdale

Heavy movable bridges have different design, operation and maintenance characteristics compared to fixed bridges. This is mainly due to the complex interaction between their structural, mechanical and electrical systems. Movable bridges are viable options in certain terrain conditions and their unique operation provides versatility. At the same time, their intricate interrelation also produces some operation and maintenance challenges. Deterioration, unexpected breakdowns, high maintenance costs and difficulty of repair are some of the issues related to movable bridges. In this paper, a Structural Health Monitoring (SHM) application demonstration on a bascule type movable bridge is presented. The ultimate objective of this ongoing project is continuous assessment of the condition and improvement of the maintenance operations. The bridge that is presented as a case study is the Sunrise Boulevard Bridge in Fort Lauderdale, Florida. This bridge has one of the most comprehensive monitoring systems in terms of the different sensors installed to monitor different phenomena. The design of the monitoring system, selection and performance of the sensors, wireless communication between the two leaves, triggering and synchronization of the data acquisition systems on both sides are presented. In addition, sample data from various types of sensors for structural, mechanical and electrical systems are shown. Finally, some challenges encountered during the field implementation are discussed. Introduction The Florida Department of Transportation (FDOT) owns and operates one of the largest numbers of movable bridges in the U.S. According to National Bridge Inventory [1], there are 146 movable bridges (3 lift type, 133 bascule type, and 10 swing type) in Florida and these are complex structures utilizing machinery to open a portion of the bridge allowing for the passage of waterborne traffic. The majority of the movable bridges in Florida are of the bascule type, having interior spans called "leaves" that rotate upward and away from the centerline of the waterway thus providing clear passage. Movable bridges are commonly used over the waterway especially in flat terrain. These bridges also present significant drawbacks and problems associated with the operation and performance. Movable bridge rehabilitation and maintenance costs are considerably higher than that of a fixed bridge. Deterioration is a concern since they are located over waterways, and often close to the coast, which constitute conditions suitable for corrosion, causing section losses. Deterioration and damage is also observed due to moving parts, friction and wear and tear of the structural and mechanical components. Fatigue can be a problem due to the reversal or the fluctuation of stresses as the spans open and close. If there are breakdowns, these cause problems for both, land and maritime traffic. Maintenance costs associated with the operation system and mechanical parts require special expertise, and may cause extensive repair work. Finally, difficulty in repair works is an issue for movable bridges. A minor or major malfunction of any component can cause an unexpected failure of bridge operation. Electrical and mechanical problems may require experts and may be difficult and time consuming to fix. Structural Health Monitoring (SHM) can be considered as an approach to continuously monitor the structural, mechanical and even electrical components of a movable bridge mainly for bridge maintenance and predicting possible problems in advance. Such a monitoring program can generate flags and warnings for maintenance to indicate a worsening condition according to industry standards, manufacturer recommendations and/or pre-set thresholds. A monitoring system is an excellent tool for real-time asset management by infrastructure owners. Infrastructure owners may use flags and warnings as a mechanism to monitor/assess maintenance performance. The data may be used by the contractors in scheduling preventive maintenance to maximize the service life of the equipment and the structure. In addition, the root causes of the structural and mechanical problems can be determined, and future designs can be improved using the information generated using the monitoring system. In this study, the writers present the monitoring design and implementation on a representative bridge in Ft. Lauderdale, Florida. First some of the critical structural, mechanical and electrical components will be described and then the issues related to these components will be defined. Finally, possible measurements to corresponding issues will be shown with some sample data. For this reason, a representative bridge which is the west-bound span of two parallel spans on Sunrise Boulevard in Ft. Lauderdale was selected. This span was constructed in 1989. It has double bascule leaves with a total span length of 117 ft and a width of 53.5 ft, carrying three traffic lanes. Each leaf is 70-ft long and 40-ft wide. The bridge opens 10 to 15 times a day. Sunrise Boulevard Bridge is shown in Figure 1. Figure 1: Sunrise Boulevard Bridge Bridge Maintenance Monitoring System (BMMS) Sunrise Bridge is the most common bascule type, with a rack-and-pinion mechanism. The bascule leaves are lifted horizontally at the point of the trunnions, which are the pivot points on the main girders. The weight of the span is balanced with a counterweight that minimizes the required torque to lift the leaf. The counterweight is made of cast-in-place concrete. In the closed position, the girder rests on a support called ‘Live Load Shoe’, or LLS, on the pier and traffic loads are not transferred to the mechanical system. The movable bridge also involves fixed components, such as reinforced concrete piers and approach spans. The counterweight of the main girder stays below the approach span deck in the closed position. When the bridge is opening, the leaves rotate upwards, and the counterweight goes down. The driving torque is generated by an electrical motor, which is then distributed to the drive shafts via the gear box. The gear box involves an assembly of gears operating similar to automobile differentials, and provides equal lifting of both sides. The drive shafts transmit the torque to the final gear called the pinion, engages the rack assembly which is directly attached to the main girder. The issues related to these components and possible measurements will be discussed in the following sections. Structural Components Main girders floor beams and stringers form the frame of the spans. They are made from both rolled and built-up sections with welded plates. The frame is generally manufactured at the shop and then installed at the site. Corrosion is a main concern on the bridge girders, especially on exposed surfaces that leads to section loss and reduced capacity. Additionally, any misalignment, bending, or deformation can also cause an increased strain on the structure. Therefore, these components were instrumented with strain gages to detect these anomalies and to trigger preventive maintenance to avoid catastrophic failures. Instrumentation with strain rosettes is also crucial because panel shear is the most likely cause of excessive stresses. Main girder failure modes will be tracked considering bending and web shear. Accelerometers can be used to register the vibrations caused by vehicular traffic. Vibration frequencies also indicate if there is change in structural system such as due to imbalance or due to span lock failure. Figure 2 shows sample data from a strain gage, strain rosette and accelerometer under traffic loading. Figure 2: Strain, rosette and accelerometer sample data under traffic Figure 3: Pressure gage and tiltmeter data during opening Strain Rosette Accelerometer VW and dynamic strain gages VW and dynamic strain gages 0 20 40 60 80 100 120 -50 0 50 Strain Gage με 0 20 40 60 80 100 120 -50 0 50 Rosette-Leg1 με 0 20 40 60 80 100 120 -50 0 50 Rosette-Leg2